We propose to use synchrotron radiation and the rotating camera on beam line 7-1 to collect a complete set of diffraction data on crystals of human DNA polymerase beta (pol _) complexed with DNA template-ddNTP mismatches. Pol _ is a eukaryotic DNA polymerase involved in the repair of damaged DNA. Specifically, pol &Beta has been shown to participate in the base excision repair (BER) pathway. Its small size (39 kD) makes pol &Beta a relativelly uncomplicated target for mechanistic studies of the nucleotidyl transfer reaction. High resolution data is needed to see the detail of conformational changes and how conformational changes promote fidelity.